Chapter 2 – Narrative review of the literature to contextualise the rationale for the introduction of dietary fat recommendations in 1977 (US) and 1983 (UK)

The diet-heart hypothesis

The review of the literature examines the origin of the hypothesis of relationships between dietary cholesterol, dietary fat (total), dietary fat (saturated), serum cholesterol and coronary heart disease (CHD). The review starts with the first use of the term arteriosclerosis [50] and ends with the first statement of the diet-heart hypothesis, which related to total fat: "The results of this survey support the hypothesis that the dietary intake of fat influences the level of the serum cholesterol, and in turn may be one of the major factors influencing the pathogenesis of coronary heart-disease" (p.1107) [5].

The focus on total fat provided the context for The Seven Countries Study, which started in 1956 [19]. The expression of the diet-heart hypothesis, as presented in The Seven Countries Study, referred to saturated, not total, dietary fat [51]. The verbatim findings of The Seven Countries Study were:

- “The incidence rate of CHD tends to be directly related to the distributions of serum cholesterol values.”

- “The average serum cholesterol values of the cohorts tended to be directly related to the average proportion of calories provided by saturated fats in the diet.”

- “The CHD incidence rates of the cohorts are just as closely related to the dietary saturated fatty acids as to the serum cholesterol level”(p.I-194) [51].

The diet-heart hypothesis became the tripartite association of saturated fat, serum cholesterol levels and CHD, although the impact of dietary cholesterol and total dietary fat were also investigated.

The review of the literature examines:

- The origin of the hypothesis of cholesterol (dietary and/or serum) as a risk factor for CHD;

- The exploration of serum cholesterol as a factor in CHD;

- The examination of dietary cholesterol as a factor in serum cholesterol;

- The exploration of dietary fat as a factor in serum cholesterol;

- The exploration of dietary fat as a factor in CHD;

- The examination of associations between dietary fat, serum cholesterol and CHD.

References are made in earlier literature to dietary fat generally. As research progressed, animal and vegetable fats became the more common terminology. Saturated and unsaturated fats were a later distinction and often used interchangeably with animal and vegetable fats, which is incorrect. All foods that contain fat, whether of animal or vegetable origin, contain saturated and unsaturated fat [52]. The transition from dietary fat to saturated fat, as the nutrient of concern, is documented.

Although not core to the diet-heart hypothesis, the origin of the hypothesis that unsaturated fat could be a potential protector is noted.

The origin of the hypothesis of cholesterol (dietary and/or serum) as a risk factor for CHD

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